Standing seam metal roofing is not only durable and weatherproof but is cheap and easy to maintain. Barbour Index and Scott Brownrigg look at how to get the best out of it
One of the most versatile and popular forms of roofing in recent years has been standing seam metal. It has established itself as a high-quality, technically sound roofing solution that also looks good. It is durable and typically lasts more than 60 years, making it ideal for commercial developments.
Standing seam metal roofing consists of metal trays that are bent up at the edges and joined at the seam. A weatherproof roof is naturally created as the joint is raised above the main roof area. Modern standing seam roofs use long extruded sections that remove the problem of having joints across the fall line found on earlier roofs. Site-based extrusion machines can produce extremely long sections.
The joints between panels offer long-lasting, consistent performance by using a double-folded profile that is formed on site, the so called “zipped” joint. The trays are fixed to the roof structure using internal brackets attached to the folded part of the section that offers several advantages. The whole roof is constructed without penetrations, the fixings can be spaced to allow for a variety of wind uplift conditions, and a cavity is created underneath for insulation.
The shape of the trays makes them rigid and self supporting, so the insulation is not crushed underneath. Cold bridges can occur only at the fixings, where thermal breaks are relatively easy to include within the fixing leg. If a section of standing seam roof is damaged, elements can be replaced easily and the weatherproofing maintained.
Check it out
1. Modern systems are more complex than they appear, so specifiers should be clear about the exact details of the system being considered. Detailed specifications vary and this will reflect on the durability of the completed roof. Check the certification and website as a first step. The leading systems have all the fittings and accessories needed. However, care is required in selecting the correct parts of the systems. Do not mix systems from different manufacturers without considering at the implications. This may invalidate any guarantee or, worse, cause a failure. Ensure you have reviewed a fully assembled sample of all significant junctions before specifying, and remember: a roof is only as strong as the weakest point.
2. Aluminium is the most common standing seam material because it is flexible, meaning that it can be curved, and it is also durable. It needs considerable amounts of energy to make but is 95% recyclable at the end of the roof’s life.
Galvanized steel, stainless steel, bronze alloy and titanium systems are available if the budget stretches. Aluminium, and the alloys used in some systems, are naturally reactive with many materials, particularly in marine environments, so ensure the system under consideration is compatible with your project. Always respect the basic characteristics of the metal in question. As the alternative metals are not that common, check the detail carefully and search out existing examples. Trade organisations can be helpful.
3. Other metals can be applied to the base aluminium sheet to give a zinc, copper natural or copper-sealed finish. The aluminium can also be anodised, or painted. A mill finish, which is a slight crazing of the top surface, is most commonly specified, and disguises minor installation damage. Other finishes, particularly bright metal ones, must be detailed and installed with care as any imperfections will show up and detract from the effect.
4. Tighter curves than the material can naturally accommodate can be achieved during the extrusion process. Double curvature roofs can also be created using tapered sheets. Ensure that the manufacturer checks curves greater than a 50 m radius because any forcing of the material will cause creases. This could cause the roof to buckle.
5. Aluminium expands twice as much as steel but less than half that of PVC, which means that allowance caused by thermal movement must be built into the system. Ensure the detailing on the perimeter of large areas can accommodate this movement. Most systems
give clear advice in this area. Specially shaped roofs will require careful analysis of the fixed points and thermal profile.
6. Although the folded seam provides good weather protection, it is not airtight. This allows the roof to breathe as any water vapour can escape through the seams. The space below the roof finish is usually filled to exclude air and avoid condensation forming on the inside face of the metal roof. However, a high-quality fully sealed vapour control layer needs to be installed below the insulation. This will minimise vapour within the insulation and can also be used as the air-seal layer. The liner tray
can be used with sealed joints but these can be distorted by walking on them, which breaks the seal. Control measures are essential to prevent this if the seal is forming the air barrier as well.
7. Steel fixing must be austenitic stainless as other steel types may induce corrosion. Ensure fixings are precisely specified and check this is being implemented
on site as substitution of fixings providing lesser performance is common and results in premature failure. Most makers also provide a range of accessories that clamp to the seams of the roof and avoid penetrations. These include access gantries and ladders, balustrades, safety systems, lightning protection and trays for the inclusion of a green roof system. Fixings will accommodate thermal movement of the roof, but the specifier should control the movement to minimise wear. This is achieved by using fixings that prevent potential movement at the centre of a roof.
Subject guides similar to this are available from Barbour Index as part of its Construction Expert and Specification Expert services. For further information, contact Barbour Index on 01344-899280 or visit www.barbour-index.co.uk